Casting system for thixoforms

ABSTRACT

Diecasting machine for production of mouldings from thixotropic metal billets, containing a sprue system which connects a cylindrical casting chamber cavity with a moulding cavity, where the sprue system has a cylindrical sprue cavity immediately adjacent to the casting chamber cavity and contains at least one sprue, and all sprues lead laterally away from the generated surface of the sprue cavity and each sprue has a concentric center line and at its end facing towards the moulding cavity has an inlet opening for introduction of the thixotropic metal alloy into the moulding cavity, and the sprue system is connected to the casting chamber cavity by a passage opening perpendicular in relation to a concentric longitudinal axis of the cylindrical casting chamber cavity, wherein each sprue has a circular or elliptical cross section with a substantially constant cross sectional area over its entire length.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/EP99/04862, filed on Jul. 10, 1999.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention concerns a diecasting machine for production ofmouldings from thixotropic metal billets, containing a sprue systemwhich connects a cylindrical casting chamber cavity with a mouldingcavity, where the sprue system has a cylindrical sprue cavityimmediately adjacent to the casting chamber cavity and contains at leastone sprue, and all sprues lead laterally away from the generated surfaceof the sprue cavity, and each sprue has a concentric center line and atits end facing towards the moulding cavity has an inlet opening forintroduction of the thixotropic metal alloy into the moulding cavity,and the sprue system is connected to the casting chamber cavity by apassage perpendicular in relation to a concentric longitudinal axis ofthe cylindrical casting chamber cavity, and the inlet openings arearranged in relation to the passage opening such that the surfacenormals of the inlet openings do not coincide with the longitudinal axisof the cylindrical casting chamber cavity.

2. Background and Prior Art

Diecasting machines for production of mouldings from thixotropic metalbillets are known in themselves. Such diecasting plants essentiallycontain a casting chamber to hold the diecasting alloy or thixotropicmetal billet, a ram moving in the longitudinal direction in the castingchamber for applying pressure to the diecasting alloy or thixotropicmetal billet, at the end of the casting chamber opposite the ram acasting chamber opening, and a sprue system comprising essentially asprue to transfer the diecasting alloy or thixotropic alloy paste fromthe casting chamber opening into the moulding cavity.

EP-A 0 718 059 describes a horizontal diecasting machine for productionof mouldings from a thixotropic alloy paste, where the diecastingmachine has an oxide scraper which is located between a semi-cylindricalarea of the casting chamber, suitable for insertion of a thixotropicmetal billet, and the moulding cavity, and which serves to prevent oxideinclusions in the alloy structure of the moulding.

DE-OS 40 15 174 describes a diecasting machine with a two-part mould forcasting of plastic or metal, where between the two mould halves isfitted a specially shaped casting holding device which can assume achanging passage cross section and in its closing position delimits atapering cross section which is smaller than the predetermined crosssection of the casting chamber opening.

The process for production of mouldings from thixotropic i.e. partsolid/part liquid metal billets is known as thixoforming. The metalbillets are all billets of a metal which can be transformed into athixotropic state. In particular the metal billets can consist ofaluminium, magnesium or zinc or alloys of these metals.

Thixoforming utilizes the thixotropic properties of part liquid and partsolid metal alloys. The phrase “thixotropic behaviour of a metal alloy”means that a correspondingly prepared metal behaves as a solid when notunder load, but under a thrust load its viscosity reduces to the extentthat it behaves in a similar way to a metal melt. This requires heatingof the alloy in the setting interval between the liquid and solidtemperature. The temperature must be set such that for example astructure proportion of 20 to 80 w. % is melted but the rest remains insolid form.

In thixoforming, part solid/part liquid metals are processed intomouldings in a modified diecasting machine. The diecasting machines usedfor the thixoforming differ in relation to diecasting machines fordiecasting metal melts for example by a longer casting chamber to holdthe thixotropic metal billet and a larger ram stroke required as aresult, and for example a mechanically reinforced design of the parts ofthe casting machine guiding the thixotropic metal alloy due to thehigher pressure loading of these parts during thixoforming.

Thixoforming takes place for example with a horizontal diecastingmachine. In this machine the casting chamber which holds the thixotropicmetal billet lies horizontal. In thixoforming a thixotropic metal billetis inserted in such a horizontal casting chamber of a diecasting machineand, by application of pressure from a casting ram, is introduced athigh speed and under high pressure into a casting mould usuallyconsisting of steel, in particular hot worked steel, i.e. it isintroduced or injected into the moulding cavity of the casting mouldwhere the thixotropic metal alloy sets. The pressure applied to thethixotropic metal billet is typically 200 to 1500 bar and in particularbetween 500 and 1000 bar. The resulting flow speed of the thixotropicalloy paste is for example 0.2 to 3 m/s and in particular 0.3 to 2 m/s.

The casting structure forming during setting of the thixotropic metalalloy in the casting mould essentially determines the properties of themoulding. The structure formation is characterised by the phases such asmixed crystal and eutectic phases, the casting grains such as globulitesand dendrites, segregations and structure faults such as porosity (gaspores, micropores) and contamination, for example oxides.

The metal billets used for thixoforming of part solid alloys have aprocess-induced fine grain which—if no grain coarsening occurs duringpretreatment of the thixotropic metal billets i.e. during heating of thebillets and their transport into the diecasting machine—recurs in thealloy structure of the mouldings. A fine grain generally improves thematerial properties, increases the homogeneity of the alloy structureand helps avoid structural defects in the moulding. Thixoforming of partsolid alloys in comparison with diecasting of metal melts also hasfurther substantial advantages. These include a significant energysaving and shorter production times as firstly the thixotropic metalbillets, in comparison with diecasting of metal melts, need be heated toa lower temperature and thus for a shorter time before thixoforming, andsecondly, in the casting mould they cool or return to a solid state morequickly, which contributes to a reduction in grain coarsening. Theenergy saving arises in particular because a majority of the melt heatand the entire superheating heat, i.e. the heat additionally supplied tothe metal alloy to achieve a temperature increase above the melt pointto ensure the liquid state of the metal alloy, and the energy forkeeping the melt warm, are no longer required. A further advantage isalso the better dimensional precision due to the lower shrinkage andproduction of mouldings close to the final dimensions, whereby themachining steps are reduced and alloy material saved. Also, theprocessing temperature is around 100° C. lower and reduces thetemperature change stress on the individual components of the diecastingmachine, which extends the tool life. The lower processing temperaturein thixoforming than in diecasting of metal melts allows the processingof alloys with a low iron content, as no alloying of the tool fromcontact melting occurs. Thixoforming also allows a better mould fillingwith fewer air inclusions.

In diecasting machines which are known from the state of the art, ametal billet in the thixotropic state, usually a thixotropic aluminiumbillet, is inserted into a casting chamber (or more precisely into acasting chamber cavity inside the casting chamber) and by means ofpressure application is pressed through a usually cylindricalconstriction at one end of the casting chamber known as the passageopening. The thixotropic material is thus sheared. The shearedthixotropic material, starting from a sprue cavity lying next to thepassage opening, is deflected into trapezoid sprues and reaches themoulding cavity of a mould. Normally the sprues are arranged atapproximately right angles to the concentric center axis of the passageopening. The arrangement between the casting chamber and moulding cavityis referred to below as the sprue system. The sprue system is used tointroduce the thixotropic alloy paste in the casting chamber into themoulding cavity of the casting mould.

The mechanical stress on the thixotropic alloy paste during its transferfrom the casting chamber cavity to the moulding cavity causes a shearliquefaction of the thixotropic alloy i.e. the thixotropic alloy becomesmore liquid as a result.

The following requirements are imposed on a sprue system forthixoforming:

a) Good filling behaviour: the sprue system must be filled as evenly aspossible over its entire cross section. In the speed range of thethixotropic alloy used, no gas or oxide inclusions may occur.

b) Good flow behaviour: the flow must be as laminar as possible to avoideddying and undesirable liquefaction of the thixotropic material.

c) Good shearing behaviour: the shear liquefaction must be ashomogeneous as possible over the entire cross section and the shearliquefaction must be kept as low as possible.

d) Low heat loss: on its passage through the sprue system, thethixotropic material should lose as little thermal energy as possible.

e) Minimum volume of sprue system: the material remaining in the spruesystem at the end of the thixoforming process is not used for thefilling process of the moulding cavity. Therefore the sprue systemshould have a minimum volume to guarantee optimum output of thixotropicmaterial into the moulding cavity.

f) Good addition behaviour: during setting of the moulding, thethixotropic material in the sprue system must remain cohesive and liquidso that firstly the pressure transfer from the casting ram to themoulding can be maintained and secondly the volume deficit on themoulding caused by setting-induced shrinkage can be compensated by theaddition of thixotropic material.

g) Good pressure transfer: the sprue system should allow as low apressure loss as possible between the casting chamber cavity and themoulding cavity.

Sprue systems which are known from the state of the art only fulfilthese requirements in part. In particular, the known sprue systems havetoo great a volume so that the output of thixotropic material permoulding can be improved substantially. Too great a volume of the spruesystem used, in particular reduces the economic efficiency of theprocess.

Another disadvantage of the known sprue systems concerns thespeed-dependent filling behaviour. The filling behaviour of a spruesystem can differ widely depending on the ram speed and startingcondition of the thixotropic billet. Thus at high ram speeds for exampleundesirable air inclusions can occur in the thixotropic alloy paste ofthe sprue system. On very rapid mould filling during thixoforming,turbulent flow conditions can occur which can lead to gas inclusions(air, separating agent or lubricant) in the mould, whereby any desirablesubsequent heat treatment, for example solution heat treatment of themoulding, is often rendered impossible. Gas inclusions close to thesurface of the moulding can for example lead to undesirable blisterformation during solution heat treatment due to the high gas pressure.

Another disadvantage of the known sprue system concerns the uneven flowbehaviour. The flow established during thixoforming after filling thesprue system with thixotropic material is uneven in most cases. It hasbeen found in particular that sudden direction changes and/or changingcross section ratios lead to local speed changes of the thixotropicmaterial. It has also been found that with angular cross sections of thesprues only part of the available cross section is effectively used forguiding the thixotropic material.

In view of the disadvantages described above of the known sprue systemsfor diecasting machines for production of mouldings from thixotropicmaterial, the inventors have faced the task of preparing a sprue systemwhich avoids the said disadvantages and which fulfils optimally therequirements imposed for a sprue system of a diecasting machine forthixoforming.

SUMMARY OF THE INVENTION

According to the invention, this is solved in that each sprue has acircular or elliptical cross section with a substantially constant crosssectional area over its entire length, and immediately next to the spruecavity has a manifold, where the part of the sprue between the manifoldand the inlet opening describes a straight tubular channel section andthe manifold is formed such that its center line has a constant bendingradius, and a tangent to the center line continued to the passageopening with the same bending radius at the passage opening runsparallel to the longitudinal axis of the cylindrical casting chambercavity, and a tangent to the center line at the end of the manifoldfacing towards the inlet opening coincides with the center line of thestraight tubular channel section.

By the design of the sprue system according to the invention, directionchanges and the associated shear liquefaction of the thixotropic alloypaste during transport from the passage opening to the inlet openingremain minimal. Each sprue preferably has a constant cross sectionalarea between the sprue cavity and the inlet opening. This keeps the flowspeed of the thixotropic alloy as constant as possible and minimises theshear effect on the thixotropic alloy.

Also, preferably the sum of the cross sectional surfaces of theindividual sprues substantially corresponds to the cross sectional areaof the passage opening. The sum of the cross sectional areas of theindividual sprues next to the sprue cavity, in a particularly preferredform, deviates by no more than ±10% from the cross sectional area of thepassage opening.

In a further preferred embodiment of the sprue system, the sprue has atits end facing against the moulding cavity a chamfer area which ends inthe corresponding inlet opening. Preferably, the sprues between thesprue cavity and the relevant chamfer area have a tubular channelsection with a circular cross section and constant radius. The channelsection between the sprue cavity and the chamfer area firstly concernsthe manifold and secondly the straight channel section between themanifold and the chamfer area of each sprue. This circular cross sectionminimises the ratio of surface area to volume. Also, the circular crosssection allows full utilisation of the available channel cross section.

Preferably, the inlet openings have an elliptical cross section. Theinlet opening arises from the plane of section of the chamfer area ofthe sprue with the widening moulding produced in the moulding cavity.With a flat moulding wall, an elliptical inlet opening therefore arises.With curved moulding geometries normally more complex planes of sectionoccur.

The chamfer area constitutes a channel-like transition area between thestraight section of the sprue with circular cross section and the inletopening. Preferably, the chamfer area along its center line has a crosssection which gradually transforms from a circular to an ever flatterelliptical cross section, where this transitional area ends in anelliptical cross section corresponding to the inlet opening. Preferably,in the chamfer area the cross sectional area is kept substantiallyconstant in size where changes of cross sectional area of up to 30% insize are included; in particular the cross section of the chamfer areaalong its center line can gradually expand or contract slightly.

In a further preferred embodiment the sprue system according to theinvention has a catchment pocket for the surface oxide layer of thethixotropic metal billet. During pretreatment, storage and the heatingprocess of the thixotropic metal billet, a metal oxide layer normallyoccurs. To avoid inclusion of such oxidic constituents in the alloystructure of the moulding, the oxidic generated surface of thethixotropic metal billet is removed usually before or in the castingchamber. Normally, an oxide layer remains on the face of the thixotropicbillet. The catchment pocket proposed in the embodiment of the spruesystem according to the invention thus allows the deposit of thissurface oxide layer in a flow-mechanically dead zone at the end of thesprue cavity remote from the passage opening. The catchment pocket isfor example formed by a cylindrical protuberance of the sprue cavity onthe side remote from the passage opening.

The sprue system according to the invention is preferably used forhorizontal diecasting machines.

Also, preferably the straight channel sections of the sprues runperpendicular to the longitudinal axis of the casting chamber cavity.The bending radius of the center line of the sprue manifold correspondsto the distance of the passage opening from a straight line containingthe center line of the straight tubular channel section of thecorresponding sprue.

According to the invention the bending radius of a center line in themanifold area is determined for example by the intersection point of theangle bisector between the longitudinal axis of the casting chambercavity and the center line of the straight part section of thecorresponding sprue with a plane through the passage opening, where thedistance between this intersection and the center point of the passageopening gives the bending radius Rk.

The transition between the casting chamber cavity and sprue cavity canbe sharp-edged or rounded. In a sharp-edged design, this transition isdescribed by the passage opening. Preferably, however, a roundedtransition is used. Here the passage opening is described by the pointat which the cross section is at its smallest or where the cross sectionassumes a constant value i.e. transforms into a sprue cavity withconstant cross section. In the rounded design form of the transitionbetween the cylindrical casting chamber cavity and the passage openingtherefore a transitional area is formed with a constantly reducing crosssection. The creation of such a transitional area causes an even sheareffect of the thixotropic alloy paste. This also avoids the break-awayof the thixotropic alloy flow from the wall of the passage opening asfrequently occurs with sharp-edged transitions and high flow speeds.

Further advantageous designs of the sprue system according to theinvention arise from the dependent claims.

The sprue system according to the invention is primarily suited forthixoforming of all metal alloys which can be transferred to athixotropic state. Preferably, the sprue system according to theinvention is used for thixoforming of aluminium, magnesium or zincalloys. Particularly preferably, the sprue system according to theinvention is suitable for thixoforming of aluminium diecasting alloys,in particular AlSi, AlSiMg, AlSiCu, AlMg, AlCuTi and AlCuZnMg alloys.

The sprue system of the invention has the following advantages over thestate of the art:

a) Minimum sprue system volume:

By the use of round sprues, the total surface is kept as small aspossible. Also, because of the optimum ratio of surface area to volume,the heat loss is minimal. Therefore less thixotropic material isrequired to compensate for the heat loss of the thixotropic alloy pastein the sprue system.

b) Good filling behaviour:

The filling behaviour of the sprue system is very good in the mouldfilling speed range—i.e. the flow speed of the thixotropicalloy—normally used for thixoforming, i.e. no air inclusions occur evenat relatively high flow speeds.

c) Flow behaviour:

The flow behaviour with a sprue already filled with thixotropic alloy isexcellent as the entire cross sectional surface of the sprue is utilisedand no flow-mechanically dead zones occur. Also, the round channel crosssection of the sprue allows the formation of a laminar flow for theentire speed range used for mould filling.

d) Adjustability of viscosity:

Due to the low shear liquefaction of the thixotropic alloy at thepassage opening and the sprue, a high viscosity of the thixotropic alloycan be retained as far as the inlet opening. At the inlet opening theviscosity of the thixotropic alloy paste required for filling themoulding cavity can be set.

e) Minimum pressure loss and good addition behaviour:

The ram pressure is transferred extremely well by the curved inletchannels according to the invention i.e. the pressure loss in the spruesis minimal and because of the hydrostatic pressure, is determined inparticular by the selected height of the corresponding inlet opening.The addition behaviour is also substantially determined by the height ofthe inlet openings due to the low pressure drop in the sprues.

DESIGN EXAMPLE

Diecasting machine with a horizontal casting chamber in which thetransition from the casting chamber cavity to the sprue cavity issharp-edged, and the sprue system has two sprues of the same dimensionseach with one chamfer area. The cross section of the sprue sectionbetween the sprue cavity and the chamfer area is circular and has adiameter of 2 R=25 mm. The bending radius of the manifold is 42.5 mm.The passage opening diameter is 35 mm. The sprue cavity is cylindricaland has a horizontal concentric longitudinal axis which also coincideswith the concentric longitudinal axis of the casting chamber cavity. Thesprue cavity has a diameter of 35 mm. The length of the sprue cavity issuch that between the two manifolds a catchment pocket is formed for thesurface oxides of the thixotropic billets, where the cross sectionaldimensions of the catchment pocket correspond to those of the spruecavity. The straight channel section of each sprue lies vertical andthus perpendicular to the concentric longitudinal axis of the castingchamber cavity, where the one sprue extends vertically downwards and theother sprue leads vertically upwards. The height of the start of thechamfer area, measured from the concentric longitudinal axis of thesprue cavity, amounts to 102.5 mm. The length of the chamfer area is 50mm. The inlet openings lie in a horizontal plane and have an ellipsoidform with a main axis length a and a secondary axis length b. The shapeof the chamfer area can be described in a Cartesian co-ordinate systemin which the x axis lies parallel to the concentric longitudinal axis ofthe casting chamber cavity, the y axis parallel to a vertical, and the zaxis also lies in a horizontal plane through the x axis such that:

x(y)=(b−R)·y/c+R

and

z(y)=(c·R2)/(b·y−R·y+R·c)

where R is the constant radius of the circular cross section spruesection between the sprue cavity and the chamfer area, b the length ofthe secondary axis of the inlet opening and c the length or height ofthe chamfer area. In this Cartesian co-ordinate system the main axis aof the inlet opening lies parallel to the z axis and the secondary axisb parallel to the x axis. The inlet openings thus have an ellipse shapewith a secondary axis diameter of 2 b=6 mm and a main axis diameter of 2a.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the diecasting machinesaccording to the invention arise from the embodiments shown in FIGS. 1to 9 and from the description of the figures.

FIG. 1 shows diagrammatically a partial view of a longitudinal sectionrunning vertically through the concentric longitudinal axis of thecasting chamber cavity of a diecasting machine according to theinvention with two sprues.

FIG. 2 shows a top view along line A—A of the diecasting machine shownin longitudinal section in FIG. 1.

FIG. 3 shows a top view along line B—B of the diecasting machine shownin FIGS. 1 and 2.

FIG. 4 shows diagrammatically a partial view of a longitudinal sectionrunning vertically through the concentric longitudinal axis of thecasting chamber cavity of a further diecasting machine according to theinvention with a single sprue.

FIG. 5 shows a top view along line C—C of the diecasting machine shownin longitudinal section in FIG. 4.

FIG. 6 shows diagrammatically a partial view of a longitudinal sectionrunning vertically through the concentric longitudinal axis of thecasting chamber cavity of a further diecasting machine according to theinvention with four sprues.

FIG. 7 shows a top view along line D—D onto the diecasting machine shownin longitudinal section in FIG. 6.

FIG. 8 shows various embodiments of a section of the upper sprue shownin FIG. 1, where this section in particular shows the chamfer area ofthe upper sprue and FIG. 8 various embodiments of this chamfer area in alongitudinal section running vertically through the concentriclongitudinal axis of the casting chamber cavity.

FIG. 9 shows the top view along line A—A of the embodiment shown inlongitudinal section in FIG. 8 of the chamfer area in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 to 9 concern for example views of a horizontal diecastingmachine according to the invention i.e. a diecasting machine withhorizontally arranged casting chamber.

FIG. 1 shows a partial view of a longitudinal section running verticallythrough the concentric longitudinal axis 1 of the casting chamber cavity12 of a horizontal diecasting machine according to the invention for theproduction of mouldings from thixotropic metal billets, where thislongitudinal section shows part of the horizontal casting chamber 10 andthe sprue system 17.

The casting chamber 10 contains a cylindrical casting chamber cavity 12which has a concentric longitudinal axis 1. The sprue system 17 connectsthe casting chamber cavity 12 with the moulding cavity (not shown). Thesprue system 17 shown in FIG. 1 has two sprues, sprue 20 and sprue 21.Sprues 20 and 21 constitute tubular structures, the cavities of whicheach have a concentric center line m₁ and m₂. Sprues 20, 21 areconnected with the casting chamber cavity 12 by means of a passageopening 14 common to the two sprues. The passage opening constitutes arotationally symmetrical opening perpendicular to longitudinal axis 1 onthe sprue-side end of the casting chamber 10.

Under pressure impacting on the thixotropic metal alloy in castingchamber 10, the thixotropic alloy paste is pressed in flow direction xthrough the passage opening 14 of the casting chamber 10 and reaches themoulding cavity of the casting mould (not shown) through sprues 20, 21.

The transition from the casting chamber cavity 12 to the passage opening14 can be sharp-edged or rounded. In a sharp-edged transition thepassage opening 14 is directly on the sprue-side end of casting chamber10. The diecasting machine shown in FIG. 1 has a rounded transitionbetween the casting chamber cavity 12 and the passage opening 14.

This gives a transition area 16 which tapers continuously in flowdirection x.

The sprue system 17 has a circular cylindrical sprue cavity 19immediately adjacent to the passage opening 14, where the crosssectional area of the sprue cavity 19 shown in FIG. 1 corresponds to thecross sectional area of the passage opening 14, and a concentriclongitudinal axis of the sprue cavity 19 coincides with the longitudinalaxis 1 of the casting chamber cavity 12. The sprues 20, 21—viewed inflow direction x—all lead laterally away from the generated surface ofthe sprue cavity 19.

Sprues 20, 21 have a circular or elliptical cross section where thecross sectional area of the sprues 21, 22 remains constant over itsentire length i.e. between sprue cavity 19 and inlet opening 35. Thesprues 20, 21 have, immediately adjacent to the sprue cavity 19, amanifold 25, 26 i.e. a curved tubular part. The part of each sprue 20,21 between the manifold 25, 26 and inlet opening 35 describes a straighttubular channel section.

As the sprue system 17 according to the invention concerns onlyarrangements in which the surface normals NE₁ of the inlet openings 35do not coincide with the longitudinal axis 1 of the casting chambercavity 12, each center line m₁, m₂ describes a curve, where according tothe invention the curved part lies at the start of the sprue 20, 21 i.e.next to the sprue cavity 19. The curved part of the center line m₁, m₂has a constant bending radius Rk₁, Rk₂. The part of the sprue 20, 21comprising the curved part of the center line m₁, m₂ is the manifold 25,26. Manifold 25, 26 is designed such that a tangent to the center linem₁, m₂, continued to the passage opening 14 with the same bending radiusRk₁, Rk₂, at the manifold start located at the passage opening 14, runsparallel to the longitudinal axis 1 of the cylindrical casting chambercavity 12.

The bending radii Rk₁, Rk₂ of center lines m₁, m₂ in manifolds 25, 26are selected such that they correspond to the distance d of the passageopening 14 from the center line m₁, m₂ of the straight channel sectionof the relevant sprue 20, 21.

In each case a straight section of sprue 20, 21 connects to the mouldingcavity-side end 73, 74 of the manifold 25, 26 so that the center linesm₁, m₂ of each sprue 20, 21, between the moulding cavity-side end ofmanifold 73, 74 and the inlet opening 35, 36, describe a straight line.In FIG. 1 the straight sections of the sprues 20, 21 stand perpendicularto the concentric longitudinal axis 1 of the casting chamber cavity 12.Consequently, the center lines m₁, m₂ of the straight sections of thesprues 20, 21 lie perpendicular to longitudinal axis 1.

The manifolds 25, 26 are also structured such that a tangent to thecurved center line m₁, m₂ at the manifold end 73, 74 directed towardsthe inlet opening 35 coincides with the center lines m₁, m₂ of thestraight channel section of the corresponding sprue 20, 21.

Sprues 20 and 21 each have at their end facing against the mouldingcavity a chamfer area which ends in the corresponding inlet opening 35,where in FIG. 1 only the chamfer area 30 of the sprue 20 is shown. Thetransition from the chamfer area 30 to the moulding cavity takes placethrough the inlet opening 35 which lies perpendicular to the center linem₁ of the straight section of the sprue 20. Therefore the surfacenormals NE₁ of the inlet opening 35 leading through the center point ofthe inlet opening 35 coincide with the center line m₁ of the straightchannel section of the corresponding sprue 20.

The sprues 20, 21 between the sprue cavity 19 and chamfer area 30, 31are described by a tubular channel section with circular cross sectionand constant internal diameter 2 R₁, 2 R₂. Radii R₁, R₂ are selectedsuch that the sum of the cross sectional areas of the two tubularchannel sections with circular cross section of the sprues 20, 21corresponds to the cross sectional area of the passage opening 14 i.e.π·R₁ ²+π·R₂ ²=π·R_(D) ², where R_(D) is the radius of the circularpassage opening 14. Consequently the sprues 20, 21 continuedtheoretically with the same bending radius to the passage opening 14,lie within the passage 14 so there is an overlapping of the sprues 20,21 with the passage opening 14.

The length of the sprue cavity 19 is formed such that the sprue cavity19 contains a catchment pocket 18 lying between the manifolds 25, 26 toreceive the surface oxides of the thixotropic metal billet. Thus, thesprue cavity 19 firstly contains the manifolds 25, 26 continuedtheoretically from the generated surface of the sprue cavity 19 to thepassage opening, and secondly the catchment pocket 18.

The inlet opening 35 shown in FIG. 1 has an elliptical form where thesecondary axis of the ellipse lies parallel to the x axis in ahorizontal plane parallel to the x-z plane, i.e. the secondary axis lieshorizontal and in a vertical plane which contains the longitudinal axis1 of the casting chamber cavity 12. FIG. 1 shows the inlet opening 35through the secondary axis of length 2 b.

The chamfer area 30 shown in FIG. 1 concerns a transitional area oflength c of the sprue 20 in which the straight section of the sprue 20with circular cross section and constant radius R₁ transforms into theelliptical cross sectional shape of the inlet opening 35. Consequently,the chamfer area 30 in FIG. 1, i.e. in a longitudinal section runningvertically through the concentric longitudinal axis 1 of the castingchamber cavity 12, has a trapezoid shape where the trapezium is formedwith equal sides and two parallel sides have length 2 R₁ and 2 b and theparallel sides are arranged at a distance c.

FIG. 2 shows a top view along line A—A of the diecasting machine shownin longitudinal section in FIG. 1. In particular, it shows the circularcontour of the catchment pocket 18, the sprues 20, 21 leadingperpendicularly away from this and the chamfer area 30 of the sprue 20.The chamfer area 30 describes a continuously expanding area of sprue 20,the cross sectional dimensions of which in this view—starting from thestraight channel section of the sprue 20 with circular crosssection—transform continuously into the elliptical cross section of theinlet opening 35. In the view shown in FIG. 2, the inlet opening has amaximum expansion of size 2 a, where a is the main axis of the ellipseof the inlet opening 35. The expansion of the chamfer area 30 shown inFIG. 2 in the direction of the inlet opening 35 is formed such that thecross sectional dimensions of the chamfer area 30 remain constant alongthe center line m₁.

FIG. 3 shows a top view of the diecasting machine shown in FIGS. 1 and 2along line B—B of FIG. 2. The ellipse shown in FIG. 3 thus describes atop view of the inlet opening 35. The inlet opening 35 lies in ahorizontal plane parallel to longitudinal axis 1 of casting chambercavity 12 i.e. in a plane parallel to Cartesian axis x-z. In a Cartesianco-ordinate system in which the x direction runs parallel to thelongitudinal axis 1 and the second horizontal axis is known as the zaxis, the inlet opening 35 shown in FIG. 3 has in the x direction asecondary axis of length 2 b and in the z direction a main axis oflength 2 a.

FIG. 4 shows a partial view of a longitudinal section of a furtherdiecasting machine according to the invention, running verticallythrough the concentric longitudinal axis 1 of the sprue cavity 12, wherein this longitudinal section can be seen part of the horizontal castingchamber 10 with casting chamber cavity 12 and sprue system 17. The spruesystem 17 contains a sprue cavity 19 and a single sprue 20.

The transition from the sprue cavity 12 to the passage opening 14 isrounded. The sprue cavity 19 adjacent to the passage opening 14 iscircular cylindrical in shape where the cross sectional diameter of thesprue cavity 19 corresponds to the diameter of the passage opening 14,and the longitudinal axis of sprue cavity 19 coincides with longitudinalaxis 1 of the casting chamber cavity 12. A manifold 25 of a single sprue20 leads laterally upwards away from the generated surface of the spruecavity 19. Next to the manifold chamber 25, the sprue 20 has a straightchannel section leading vertically upwards, to which is connected achamfer area 30. In the view shown in FIG. 4, the chamfer area 30 tapersconically upwards and ends in the inlet opening 35. The cross sectionalsurface of the sprue 20 over its entire length, i.e. between the spruecavity 19 and the inlet opening 35, substantially corresponds to thecross sectional area of the passage opening 14. The length of the spruecavity 19 is such that a catchment pocket 18 is created to hold thesurface oxide of the thixotropic alloy paste. In the embodiment shownhere, the length of the sprue cavity 19 corresponds to the distance ofthe passage opening 14 from a tangential plane standing normal tolongitudinal axis 1 and lying at the straight section of sprue 20 on theside remote from the casting chamber cavity 12.

FIG. 5 shows a top view along line C—C of the diecasting machine shownin longitudinal section in FIG. 4. Here, in addition to the catchmentpocket 18, seen circular in this top view, is shown the sprue 20 withits chamfer area 30. The sprue leads vertically upwards. The chamferarea 30 in this top view concerns a continuously expanding area of thesprue 20 where the shape of the chamfer area 30 is selected such that ininteraction with the view shown in FIG. 4, the cross sectional surfaceof the chamfer area 30 remains constant over its entire length.

FIG. 6 shows diagrammatically a partial view of a longitudinal sectionrunning vertically through the concentric longitudinal axis 1 of thecasting chamber cavity 12 of a further diecasting machine according tothe invention. The transition from the casting chamber cavity 12 to thepassage opening 14 is rounded. Next to the passage opening 14 is acircular cylindrical sprue cavity 19, the cross sectional diameter ofwhich corresponds to the diameter of the passage opening 14, and thelongitudinal axis of which coincides with the longitudinal axis 1 of thecasting chamber cavity 12. Four manifolds 25, 26, 27, 28 lead away fromthe generated surface of the sprue cavity 19, where in FIG. 6 i.e. in avertical plane along longitudinal axis 1 only two manifolds can be seen,namely manifold 25 of a sprue 20 leading vertically upwards and manifold26 of a sprue 21 leading vertically downwards. To the manifolds 25, 26are connected straight channel sections leading vertically upwards anddownwards respectively of sprues 20, 21 with circular cross section. Thechamfer areas 30, 31 connected to these straight channel sections havein the view shown in FIG. 6 a conically tapering cross section. Betweenthe manifolds 25, 26 is enclosed a protuberance of the sprue cavity 19,the so-called catchment pocket 18.

FIG. 7 shows a top view along line D—D of the diecasting machine shownin longitudinal section in FIG. 6. In this top view can be seen foursprues 20, 21, 22, 23 arranged in a cross shape. The concentric centerlines (not shown) of these sprues 20, 21, 22, 23 enclose a right anglein this top view. In the center of this top view is the circularcatchment pocket 18. To the straight sections of sprues 20, 21, 22, 23,leading away in a cross-shape from the catchment pocket 18 in thecenter, are connected the corresponding chamfer areas 30, 31, 32, 33.These chamfer areas 30, 31, 32, 33 describe the transition area betweenthe straight sections of sprues 20, 21, 22, 23 and the correspondinginlet openings 35, 36, 37, 38. The chamfer areas 30, 31, 32, 33 in thistop view concern a continuously expanding area of sprues 20, 21, 22, 23,where the shape of the chamfer areas 30, 31, 32, 33 is selected suchthat in interaction with the view shown in FIG. 4, the cross sectionalarea of each chamfer area 30, 31, 32, 33 remains constant over itsentire length. All four sprues 20, 21, 22, 23 have the same form andsame dimensions. Also the sprues 20, 21, 22, 23 are formed such thattheir cross sectional area remains constant over its entire length i.e.from the sprue cavity 19 to the corresponding inlet openings 20, 21, 22,23. The surface normals NE₁, NE₂, NE₃, NE₄ on the inlet openings 35, 36,37, 38 lie parallel to the center lines of the straight sections of thecorresponding sprues 20, 21, 22, 23. Adjacent surface normals NE₁, NE₂,NE₃, NE₄ enclose a right angle between them.

FIG. 8 shows various embodiments of an extract of the upper sprue 20shown in FIG. 1, where this section in particular concerns the chamferarea 30. Consequently, FIG. 8 shows various embodiments of the chamferarea 30 in a longitudinal section running vertically through theconcentric longitudinal axis 1 of the casting chamber cavity 12. Here,the inlet opening 35 remains unchanged for all embodiments of thechamfer area 30. It is essential for the embodiments of the chamfer area30 shown with the chamfer walls e, f, g that the chamfer area 30, as atransitional area between the straight channel section of the sprue 20and the inlet opening 35, has the same cross sectional area throughoutover its entire length and for all embodiments of the chamfer walls e,f, g. In the longitudinal section according to FIG. 8 the chamfer wall f(solid line) has the shape of a trapezium with equal sides andcorresponds to the view of the chamfer area 30 shown in FIG. 1. Thechamfer wall e has a form curving continuously inward, and the chamferwall g a form curving continuously outward.

FIG. 9 shows the top view along line A—A of the embodiments of thechamfer area 30 of FIG. 1 shown in longitudinal section in FIG. 8. Theinlet opening 35 again remains unchanged for all embodiments of thechamfer area 30. To fulfil the requirement for a cross sectional areaalong the chamfer area 30, where this requirement applies for allembodiments of the chamfer area 30, the chamfer walls e, f, g in the topview according to FIG. 9 must have a greater cross section, the smallertheir cross section in the longitudinal section in FIG. 8. Consequently,the chamfer wall e in FIG. 9 has a trapezoid shape whereas the chamferwall f compared with the chamfer wall e is curved continuously inwardsand the chamfer wall f compared with the chamfer wall e in the top viewshown in FIG. 9 overall has a smaller cross section. The chamfer wall gin comparison with the chamfer wall f has a stronger inward curvature sothat its cross section in the top view shown in FIG. 9 overall issmaller than the chamfer wall f.

What is claimed is:
 1. Diecasting machine for production of mouldingsfrom thixotropic metal billets, which comprises: a sprue system whichconnects a cylindrical casting chamber cavity with a moulding cavity,where the sprue system has a cylindrical sprue cavity immediatelyadjacent to the casting chamber cavity and contains at least one sprue,and said at least one sprue leads laterally away from a generatedsurface of the sprue cavity and wherein each sprue has a concentriccenter line and at its end facing towards the moulding cavity has aninlet opening for introduction of the thixotropic metal alloy into themoulding cavity; wherein the sprue system is connected to the castingchamber cavity by a passage opening having a surface normal andperpendicular in relation to a concentric longitudinal axis of thecylindrical casting chamber cavity, and the inlet openings are arrangedin relation to the passage opening such that surface normals of theinlet openings do not coincide with the longitudinal axis of thecylindrical casting chamber cavity; and wherein each sprue has acircular or elliptical cross section with a substantially constant crosssectional area over its entire length, and immediately next to the spruecavity has a manifold, where a part of the sprue between the manifoldand the inlet opening describes a straight tubular channel section andthe manifold is formed such that its center line has a constant bendingradius continued to the passage opening and a tangent to the center lineat the passage opening runs parallel to the longitudinal axis of thecylindrical casting chamber cavity, and a tangent to the center line atthe end of the manifold facing towards the inlet opening coincides withthe center line of the straight tubular channel section.
 2. Diecastingmachine according to claim 1, wherein at least one sprue at its endfacing towards the moulding cavity has a chamfer area which ends in thecorresponding inlet opening and the sprue between the sprue cavity andchamfer area is described by a tubular channel section with a circularcross section and constant diameter.
 3. Diecasting machine according toclaim 2, wherein the chamfer area describes a channel section which onthe side next to the straight tubular channel section of the at leastone sprue has a circular cross section, and the cross section of thechamfer area starting from this circular cross section transformscontinuously and constantly into a cross sectional form of the inletopening of the corresponding sprue.
 4. Diecasting machine according toclaim 3, wherein a cross sectional area of the chamfer area of a sprueremains substantially constant along its center line and nowhere variesby more than ±30% of the cross section of the straight tubular channelsection next to the chamfer area of the corresponding sprue. 5.Diecasting machine according to claim 1, wherein a surface normal theinlet opening of at least one sprue is arranged perpendicular to thecenter line of the straight tubular channel section of the correspondingsprue.
 6. Diecasting machine according to claim 1, wherein the centerlines of the straight tubular channel section of the sprues enclose aright angle with the longitudinal axis of the casting chamber cavity. 7.Diecasting machine according to claim 6, wherein the bending radius ofthe center lines in the manifold of a sprue corresponds to a distance ofthe passage opening from a straight line containing the center line ofthe straight tubular channel section of the corresponding sprue. 8.Diecasting machine according to claim 1, wherein the longitudinal axisof the casting chamber cavity lies horizontal.
 9. Diecasting machineaccording to claim 1, wherein between the casting chamber cavity and thepassage opening is arranged a transition area with a constantly taperingcross section starting from the casting chamber cavity.
 10. Diecastingmachine according to claim 1, wherein a sum of the cross sectional areasof the individual sprues substantially corresponds to a cross sectionalarea of the passage opening.
 11. Diecasting machine according to claim10, wherein the sum of the cross sectional areas of the individualsprues lying at the sprue cavity deviates by no more than 10% from thecross sectional area of the passage opening.
 12. Diecasting machineaccording to claim 1, wherein a longitudinal axis of the cylindricalsprue cavity runs parallel to the longitudinal axis of the castingchamber cavity, and the cross sectional area of the sprue cavitysubstantially corresponds to the cross sectional area of the passageopening.
 13. Diecasting machine according to claim 1, wherein the lengthof the sprue cavity is selected such that between the manifolds isformed a catchment pocket for holding the surface oxide of thethixotropic metal billet.
 14. Diecasting machine according to claim 1,wherein the inlet openings have an elliptical cross section.